Magnetically controlled rectifier-tube



Oct. 28, 1952 J BOYER 2,616,064

MAGNETICALLY CONTROLLED RECTIFIER-TUBE Filed March 30, 1951 SHEETS-SHEET 2 3o ,40 0.0. Load v I v WITNESSES: INVENTOR John l Boyer.

BY 7 0? A 9 ATTORNEY Patented Oct. 28, 1952 MAGNETIGA'IJ'LY CONTROLLED RECTIFIER-TUBE JohnLrBoyer,Bittsburgh, Pa., assignor to Westinghouse.ElectrieCorporation, East Pittsburgh, Pa.,, a corporation, of Pennsylvania.

Application March-30, 1951, Serial No. 218,368

12.01aims. 1:

Myinvention. relates to -deiayedt-firingjgaseousr discharge rectifying tubes or devices, in which electromagnet-means are. provided for at times establishing anarc-blocking. magnetic field trans.- versely across: an; intermediate: arcing-space. through which the, arc must pass in playing. b.e.-- tweenthe anode and the cathode of the-tube.

Magnetically controlled rectifying tubes have been. known before, in which a. discharge=blocking electromagnet hasbeen. energized. with; a pulsating unidirectional current for permitting; an are or discharge. to form withinthetube only during the periods when: the electromagnet was deenergized, thus controlling. the delayed firing of thetube. However; these-magnetically controlled. tubes. as previously known, have not. operated satisfactorily, and in: fact. the operation has; beenso. poor that most ofthese tubes are no'longer listed by theprincipal tube-manufacturers; most controlled. tubes. having. been. long since. stand:- ardized, in favor of the grid-controlledtypes; The principal disadvantagebf. magnetically controlled. tubes, as. heretofore known,.has. been, due to. thefactthat the time. constantof. the? electremagnet is ingeneral of. the same order as, and usually considerably larger than, the time-delays which. are involved in the. controlled; delayed fir.- ing of the tube, wherebythe. conductiveperiods of the tube. shall. be. timed, at. predetermined phase-angles after. the commencement of. the.

positive. halt-wavev of voltage in which. the tube is to. conduct. current.

It is the principalobject. of. my present inven-- tion togo backto the. discarded idea of magi-- neticallycontrolled tubes, and-to prov-idemeanswhereby the. strength. of the electromagnetfieldz is reducedifroman. efiective. arc-blocking value to. substantially zero at. a. rate.v of. decrease, which is considerably higher than therate. of decrease: of the voltage. at the end ofv each. half-cycle in the main alternating-current circuit from which. the rectifier isv energized. There are many means.- for bringing about thissharpcutoff of the. mag,- netic field of the electromagnet, most.- of these. means. being adaptations. of means. and. circuitswhich are already known in other uses. cr'applications.

My invention consists partly in various adaptations. of various quick-cutoff. means, but more basically, my invention relates tov the-newmethod: ofm-agnetic control wherein a quick-cutoff means. or circuit. is interposed between the. alternating or pulsating-current source of; energy: for the; electromagnet and the: electromagnet. itself, so

as to; cause: the: electromagnet to. be. able tolose- .r-

H tubes.

its flux at a.much higher percentage-rate than thepercentage-rateof. changeof the line-voltage at the. beginning: or: end of. any half cycl'el In this way, I give my magnetically controlled .tubes the sharply defined, and definite, delayed-firing control; which occurs at the: instant when: the electromagnet loses its flux, thereby: ranking the electromagnetic control in efiiciency and'dependa ability comparable. to the. standard; grid-controlled tubes.

While my inventionis: applicable;to:v any; delayed-firing rectifying tubes, which may be: of: either the; hot-cathodetyDe. or. theca-thod'e-pool; type, a probably morenimportant. field ofifapplii cation of the invention, is; in; connectionwith single-anode alkali-metal; tubes, either. of; the: cathode-pool type: or of. the hot-cathode type, wherein the discharge-metal is a metal chosen from the group comprising. the; three; heavier; stable light metals; of; the' alkalia-meta-l group, comprising cesium, rubidium and: potassium; as; more particularly described and claimed: in: an' application. of, Golaiacoand. myself, Seriah No. 144,354, filed February" 15; 195.0, wherein; a. hotcathode type of alkali-metal tube is. described; and in my application,,serial. N0:.,20.6',4.'345..fi19.(15 January 17,1951, in which a-pooltype of. alkali,- metal tube is disclosed.

Electromagnetic. delayed firing control. is par ticularly necessary or, advantageous: in: alkali-r metal tubes, because of the" unusually greatzdifit culties which are encountered in the; provision: of a. successful electrostatic grid for: controllin the initiation of thecurrentscarrying arcs-insuchz To be. effective, a grid, irran. alkali-metal; tube, should be made of a metal whose workiunc tion, is. lower than. the ionization: potentialotthe alkali-metalwhich is. usedin the tube; andLthBa grid should also, be kept at a, relatively; low:

temperature. For example, the ionization potene mm of cesiumis 3.9 volts, and if the-metal from: which the grid is constructed has. awork. tuner" tionfhigher than 3.9 volts, the grid acts as. an.

electron-emitter, even at temperature. as lowas.

. 320 C., and. thus loses its. ability to prevent, the: initiation of. the current-carrying arc; in. the;

tube. Metalsv Whose Work; function is-lower'than.

3.9 volts, such as zinc (3.3 volts), thorium. (1314':

volts) calcium. (3.2volts)", magnesium (2.4 volts),

. and others, are relatively impractical, for grid The other-- alkali-metals, rubidium and potassium, also have low ionization potentials, which are 4.2 and 4.3 volts, respectively. In order to obtain the same advantages which are provided by grid-control in other tubes, it is particularly desirable, therefore, in an alkali-metal tube, to use an electromagnetic field-control, with the improvements which are embodied in my present invention.

With the foregoing and other objects in view, my invention consists in the circuits, systems, combinations, structures, parts, and methods of design and operation, which are shown in the accompanying drawings, wherein:

Figure l is a somewhat diagrammatic and simplified vertical sectional view of a hot-cathode alkali-metal tube embodying my invention, the cross-section being taken on the line 1-1 of Fig. 2;

Fig. 2 is a plan view or the electromagnet, showing a transverse section of the tube on the line IIII of Fig. 1;

Fig. 3 is a curve-diagram showing the kind of delayed firing which is attained in a common type of three-phase assembly of rectifier tubes, to which my invention is applicable;

Fig. 4 is a curve-diagram showing the squaretopped alternating voltages, and the squaretopped pulsating unidirectional currents, which are obtained in one phase of the magnetic control -circuits which are used in the threephase system of Figs. 3 and 5;

Fig. 5 is a diagrammatic view of circuits and apparatus showing both the main power-circuits and the electromagnet control-circuits of an exemplary form of my invention as applied to a three-phase rectifier-system, the tubes being diagrammatically indicated as dumbbell-shaped pool-type tubes, which are intended to be symbolic of any gaseous-discharge tubes to which the invention may be applied, and

Figs. 6, 7 and 8 are wiring-diagrams showing alternative forms of electromagnet controlsystems.

The alkali-metal tube which is shown, by way of example, in Fig. 1, is of the hot-cathode type, with an air-cooled anode. The tube comprises an evacuated enclosure-means or container, most of which is made up of a metal cathode-portion 9 having cylindrical side-walls 9a, surmounted by a constricted portion 91) which provides an intermediate arcing space of restricted cross-sec-' tion, through which the arc must play in carrying the load-current of the rectifier. The cathode container-portion 9 has a bottom part 9c which is provided with a centrally disposed upstanding reentrant tube-portion 9d having a closed upper end 9e.

The upper end of the evacuated container of the tube in Fig. 1 consists of a downwardly extending cup-shaped metal anode portion l9,

- which is joined to the top of the cathode-portion 9 of the container by means of a cylindrical glass or other insulating member H, and metalspinnings l2 and I3 which are connected sealtight between the cylindrical insulator I I and the anode and cathode parts H! and 9, respectively. The anode is illustrated as being of a type which is cooled by means of cooling-fins M which are carried by a massive copper cup-member l5 which fits down into the cup-shaped anodemember I0, in good thermal and electrical contact therewith, so that the anode is directly cooled by radiation to the ambient atmosphere.

The lower cylindrical walls 9:: of the cathodeportion 9 of the evacuated container are alsocooled by being exposed to the ambient atmosphere, the cooling-rate of this cathode cylindrical wall 9a being such that said wall is preferably cooled to a lower temperature than either the anode It or the insulating seal |2-I l-I3, so that the metal-vapor condensation occurs only on the cylindrical cathode-walls 9a, at a condensation-temperature which is usually of the order of from to 250 C., more or less. The tube contains a small quantity of a dischargemetal selected from the group comprising cesium, rubidium and potassium, the quantity of this metal being too small to be shown conveniently in a drawing of the small scale of Fig. l.

The re-entrant cathode-tube M is heated to a temperature of the order of 700 or 800 C'., by means of a cathode-heater l6 which is disposed within said re-entrant cathode-tube 9d, being illustrated as being held hermetically sealed within said re-entrant cathode tube, by means of a vacuum-tight bottom-plug I! having an insulating seal l8 for bringing out the heater-lead l9.

It is practically necessary, in the illustrated type of hot-cathode alkali-metal tube, as shown in Fig. 1, for the hot-cathode surface-area to be quite large, without permitting the cathode to occupy an unreasonably large space. To this end, the heated re-entrant cathode-tube 9d is provided with a large number of outwardly extending cathode fins 21, which are connected to the outer surface of the re-entrant cathode-tube 9d, in good thermal and electrical contact therewith. These cathode-fins 2| practically fill the lower cylindrical portion 9a of the cathode-part of the container. The heat-loss from the cathodefins 2| to the cooled parts of the cathode container-portion 9 is usually reduced by means of thin spaced metal heat-shields 22 and 23, which are suitably supported within the container, as described and claimed in the previously-mentioned copending application of Colaiaco and myself.

In accordance with my invention, the constricted portion 91) of the envelope, between the anode I0 and the cathode-fins 2|, is placed between the poles of an electromagnet 25, which is provided with a laminated core in order to decrease its time-constant which is eliective during the increasing and decreasing of its flux. The disposition of the magnet-poles on opposite sides of the restricted section 911 is shown in Fig. 2. The electromagnet 25 is provided with an energizing coil 26, which may be energized in any one of a number of difierent ways, some of which are shown in Figs. 4 to 8, by way of example; the underlying principle of the coil-energization being that the electromagnet-coil 26 should be connected in such a circuit that the electromagnet reduces its magnetic field to zero very rapid 1y, when the coil is deenergized, more rapidly than the percentage-rate of decay of the alternating-current line-voltage near the end of any half-cycle. To avoid short-circuiting the magnetic flux of the electromagnet 25, I prefer to use stainless steel or other substantially nonmagnetic material for the evacuated container 9, or for the constricted portion 5b thereof, as is well known in magnetically controlled aredevices.

Fig. 5 shows one way of energizing the electromagnet-coil 25, in accordance with the invention. Fig. 5 shows a simple three-phase rectifiersystem. in which a direct-current load 30 is energized through three magnetically controlled rectifiers 3|, 3'2 and 33 from the star-connected secondary winding 34 of a transformer having a primary" winding}. which; is. energized; from: a.

three-phase.-.supply-linetii: The rectifiers: 34-..3-2 and! are diagrammatically indicated; inF-ig; 5; inta manner: which is-intendedi to; represent any magnetically controlled. gaseousrdischarge-rectifier; whether of-Jthe: filamentary-cathode or vapor-arc: type; and-if vapor-arc, whether; mercuny-vapor,. alkali-emetalr vapor; or: other: vapor; and alkali-metalf vapor, whether: of the.- pool type. or: of the: hotrcathode': type: which; is; shown incl; .In orden to. show: something? which. is representative of. a; rectifier: in Fig: 5;.I have". in? dicated thGFI'GCfifiGIS 31', 32zand 33 as'ifitheyiwere single -anode;cathode-pool. rectifiersgwhich would necessarily; therefore; be provided with some: sort ofiim'akeralive device: or keepealivedevice; which is: diagrammatically indicated: at. 31-...

Eaclzu of: the: rectifiersdiy. 323' and: 33? in. Fig; 5 is, illustrated. as: comprising; a restricted; inter;- mediate section. which. is; under the influenceot a; magnetic: field. whicl'e may be. produced by the: electromagnet'rcoil; Z6:

In. Fig. 5', in; accordance: with. a. welleknown rectifier-circuit; the:- anodes: of the respective rectifiersr' areenergized. from the. respective: terminals: of. the: st'areconne'ctedi secondary winding 31!, as. indicatedi at: 38; while. the three cathodes are connected. together in: a common: circuit. 36 which; constitutes; the positivesupply-lead for thedirectt-current:loadzwa The negativezreturnlead .for." the direct-current; load is: brought back to; the; secondary.- neutral, as, indicated by the conductor? 49 l.

all of: the. farms of embodiment of my invention; the. electromagnetiwhich. is. diagrammatically' representediby: the: coil Z8318 energized by a. special; energizinge-cincuit' which: gives the electromagnetaa. low tii'neeconstant, or" a. steep'- wave-front. characteristic, by: which i? mean the ability' to decrease its. flux; from. a dischargeblocking: field-intensity to'. substantially zero: in a'. timerwhichcis small compared: to the percent-'- agez-rates of: increase: of? the. line-voltage at theend. of: each positivea halt-cycle..

In the particular." form; of; embodiment of-"my invention: which is shown in Fig. 5; the steepwave-front; electromagneteenergizing' meansincludesethercombinatiorr. of; an alternating cup rent electromagnetesupplying'. circuit: 41 a-phase controlling: means; such. as a. serially connected variable: inductanceqd'ig. and. a square-wave voltage-producing; means: energized thereby; said square-wave voltage-producing means being illustrated;. in; Fig;v as; a. capacitor M and a saturableinductor: 45;..both connected. in: parallel across: the phase-controlled electromagnet-sup-- plyingcircuit; The; saturableinductor 45 has a magnetic circnitzof. a type". which: has a sharply defined: saturation-point. in its. saturation-curve; and the: relative sizes; of". the capacitor 44" and the saturable: inductor' liiz are such that the inductor requires something. like. (usually a little less than) ahalf-cycle; of: the. electromagnet supplying circuit-, to: change it's; magnetization from. substantially zero.- to; the: SfiztlllfifiOllr-IIOlllt' in. either polarity.

Thus,- when' the saturating inductor begins. to. saturate,-. it. completes. its.- saturation very rapidly therebyreducing its impedance to an extremely low value. which short-circuits the capacitor M, thereby drawing a still larger exciti'ng-current. and reducing the terminal-voltage of. the capacitor to. zeroin. an extremely short-time;v This is timed so that it occurs at,

orijust. before, the. occurrence of a. zero-voltage condition. in; the: circuit; which: supplies; the capacitor- 4.4 withline-frequencyalternatinge current energy. The line-frequency: supply voltage. passes. through. its zero point-,fiandi. begins to build; up in the; negative direction; at. about the time when: the capacitor overshoots, in. its discharge-action; and builds up: a: charge; in: the opposite. direction. so that: the now: demagnetized or unsaturated; inductor; 4.5. begins. to build. up its: flux in; the opposite direction, repeating the half-cycle saturation-process: The result. isa fiat-wave voltage-curve; across the terminals:- of the. capacitor 44., somewhat as indicated in, Fig; 4; by the flat-topped. alternating-current. wave 41-48-49- The" electromagnet-ccil; 21%, in Fig. 5:, is. con-.- nected; across: the square-wave; voltageeproducing terminals. of. the capacitor by'a coilr-energizing circuit which serially: includes ahalf-waye redid-.- fier 5| of a type which becomes. conducting. substantially as: soon. as its impressed", voltage becomes: positive in' the. conducting; polarity; In: this way; the coil: receives a: unidirectional: puleating? energizatiom. or; square-toppedmalf-waves of only onepolarity;

The: electromagnet. would. still have far' toolong a: time-constant, even when energized. from; a; substantially square-topped: voltage-wave; if some special: circuit-connectionwere: not. used; to reduce this time-constant; In-1iig:..5,..this means, which is: used for reducing; the; time-constant of; the" electromagnet',. is: a; large: resistance; 53:, which iSiCOllIlECtQd-ihiSGlifiS with the electromag; net-coil 28', or? built into: the coil.;itself, ,sothatv the:,seriallyinc1uded* resistance, in: the; coilieena ergizing circuit; is. at least. about. twice 'aslarge as; the' supply-frequency inductance of the: coil- 26. By making the coil-circuit resistance: 53- as large asmay? be. necessary; the: time-constant oithe:e1ectromagnet'v can. be."- made; quite: small; so. that. itsv current; and, hence its. magneticafiux; willa follow substantially the: same. wave-form as: the impressed.v voltage; whichzis. the voltage across the: terminals: of. the: capacitor M. In Fig; 4, therefore; the? heavy-line flatrtbpped intermittent. or. pulsating-wave: current of the electromagnet-coih 2%5- can; be; represented; by: a curve; 4:15.5--4l;, wherein the; zero portion 55 of; thecurve represents the coil-current during the: nonconductingr periodof: the: serially con:- nected rectifier; 5L. It; will, be; noted: that? the half-cycle: current-pulsations M and; 45? havea steeperethan-sinusoidala change-rate at; both the beginning and: the; end. of: each. halt-wave current-pulse;

It. will be: noted; thatrthe. squareewavezvol'tagee generation, which is. produced. in. by" the parallelrconnected:capacitor 4.4? and? the;-sa-turable inductor?45;,uperateszby-a tuningtor adjustmentof. the relative-- magnitudes of: the; capacitive: inipedance: or the charging-current-which is: drawn by the capacitor- 44 and the: nonsaturated; in.- ductive: impedance or the: magnetizing current. which is drawn by the, inductor 4&- before its saturation-point has been. reached... order to keep this combination of capacitive. and:induc.-- tive reactancesropera-ting properly; it is necessary that the: amount of; energy which restored: bycthe capacitor-d 4" during; each half-cycle. shall: be large. as compared. to" the}. amount ofv energy which. is drawn? on? by; the half-wave: coil-circuit: of the electromagnet-coil: 26.. Preferably; this capacitor-stored energy; during: each. half; cycle. should: be at least four times; larger: than. the. amount of: energy which is. drawnnby the. coir-energizing;

7 circuit during each half-Wave, while the coil 26 is receiving energy from the terminals of the capacitor 44.

While any suitable time-phase adjustingmeans may be used, as shown at 42 in Fig. 5, the most practical form of device for this purpose is a phase-shifting reactor 42, which is a controllably saturated inductance having a unidirectionally energized saturation-coil 62 which is supplied with a controllable direct current, as by means of a battery 63 and a rheostat 64, or any other suitable control-means for varying the degree of saturation and hence the inductive impedance of the coil 42. This combination of a variable-impedance coil 42, in series with the capacitor 44, makes a very convenient phase-controlling means, for varying the time-phase of the alternating-current voltage which is impressed upon the terminals of the capacitor 44.

Reference to Fig. 3 will show the manner in which a three-phase rectifier of the type shown in Fig. 5 will operate. The sine waves of the three phases of the supply-voltage are shown at 1, 2 and 3, while the conductive periods of the respective tubes are indicated by the heavyline portion of the curve, which can thus be taken to represent the direct-current outputvoltage of the three-phase rectifier-assembly. It

will be noted that each rectifier becomes conducting at a certain time-phase angle 9 after the anode of that rectifier becomes positive with respect to the common cathode-circuit. By a variation of the magnitude of this firing-delay angle 9, the average value of the direct-current output-voltage can be controlled in a well-known manner, this being the object of controllably delayed firing.

Fig. 4 is related to Fig. 3 in showing the relative time-phase of the electromagnet current 4'|-'-55-49 which energizes the electromagnetcoil 26 for the phase-1 rectifier 3|, so that said phase-1 rectifier begins conducting at the terminal of the delayed-firing time-period or angle 6. Thus, as long as the electromagnet coil 26 is energized, as indicated by the heavy-line, current half-wave 41 in Fig. 4, the phase-1 rectifier 3| can not become conducting, even after its anode becomes more positive than its cathode, as it does at the crossover point 65 which marks the beginning of the time-period during which the delayed-firing angle 9 is being counted. As soon, however, as the electromagnet-coil current becomes zero, as indicated by the point 55' of the curve in Fig. 3, this phase-1 rectifier 3| fires, and it continues to carry current until its current is transferred over to the next rectifier 32, as indicated by the heavy portion of the curve 2 in Fig. 3. It is to be noted that the very steep current-reduction vertical line, which defines the firing-point 55' in Fig. 4, is an important characteristic of my invention, by reason of which it becomes possible for me to successfully use magnetic firing-control in a manner which gives definite, positive, and accurate control of the firing-angle 9.

In Fig. 5, because of the variable time-delay or phase-angle which is introduced by the variable inductor 42 it is necessary to select, for the phase-1, electro magnet-supplying circuit 4|, a line-frequency voltage-source which leads the phase of the anode-voltage l which is applied to the phase-1 rectifier 3|. Any suitable advanced-phase electromagnet-energizing voltage could be used; by Way of example, I have 8 chosen a voltage which leads the phase-1 anodevoltage by electrical degrees, as indicated in Fig. 5 by the conductor 68, which is used to energize an auxiliary transformer 69 which energizes the electromagnet-supplying circuit 4|.

The electromagnet-supplying circuits which are shown in Fig. 5 are only exemplary of a number of different kinds of circuits which will perform the general function of causing the electromagnet to have a steep-wave front dis charge-characteristic, whereby it decreases its magnetic flux or field, from an effective arcblocking value to substantially zero, at a rate of decrease which is very fast and sharp, or considerably higher than the percentage-rate of decrease of the voltage at the end of each halfcycle in the main alternating-current supplycircuit. Several alternative forms of equivalently functioning electromagnet-supplying circuits are shown in Figs. 6, 7 and 8, respectively.

In Fig. 6, the electromagnet-supplying circuit 4| is shown as being connected, as before, to a capacitor 44, through a variable inductor 42, but the means for causing the capacitor-terminals to produce a succession of substantially squaretopped current half-waves 41, 49 in the electromagnet-coil 26 is diiferent, in Fig. 6. In Fig. 6, a transformer II, or other midtapped inductance-device, is connected across the terminals of the capacitor M, so that th midtap 12 produces a terminal point for a common return-path circuit 73 for two alternately pulsating unidirectional-current circuits 14 and 15 which are energized from the respective terminals of the capacitor 44. Each of the pulsating unidirectionalcurrent circuits 14 and 15 includes a rectifier 5|, which is similar to the rectifier 5| of Fig. 5. At least one of these circuits includes the electromagnet-coil 26 which is to be controlled; or if there are two rectifier tubes, such as 3| and 3|, which are to be energized with their anodes out of phase with each other, the electromagnet-coil 26 of one tube can be energized from one pulsating-current circuit, 14, while the electromagnet-coil 26 of the other tube is energized from the other pulsating-current circuit, 15.

In the form of my magnet-energizing means, as shown in Fig. 6, I use a large choke-coil or inductance 76 in the common return-path circuit-part 13 for the two pulsating-current circuits 14 and 75, this inductanc being large enough to keep the unidirectional current in this common return-circuit path substantially continuous and constant in magnitude, being preferably substantially ripple-free. Because this inductance 16 is in a direct-current circuit, it must have an air-gap magnetic core, as indicated at 77. This inductance 15 thus constitutes a means for providing whatever large voltage-impulses may be necessary, first in one polarity and then in the other, in order to force the currents which flow in the coils 26 and 26', during alternate half-cycles, to increase practically instantaneously to their full value, at the beginning of each half cycle, to thereafter maintain that value constantly, without change, throughout that half-cycle, and then to decrease substantially instantly to zero, while the current in the other coil is simultaneously increasing, substantially instantaneously, to its full value. In this way, the necessary commutating voltages, or currentforcing voltages, are provided, whereby squaretoppe d current-pulses are produced in the respective electromagnet-coils 25 and 26' of Fig. 6.

In Fig. 6, as in Fig. 5, it is necessary to have voltage becomes positive phase-controlling means associated therewith, a substantially square-wave voltage-producing means energized thereby, and a coil-energizing circuit energized from said square-wave voltageproducing means, said coil-energizing circuit comprising a serially connected half-wave rectifier and serially included resistance which is at least about twice as large as the supply-frequency inductance of the coil, said serially connected half-wave rectifier being of a type which becomes conducting substantially as soon as its impressed in the conducting polarity;

- 2. The invention as defined in claim 1, characterized by said square-wave voltage-producing means comprising a capacitor and a saturable inductor both connected in parallel across the phase-controlled electromagnet-supplying circuit, said saturable inductor having a sharply defined saturation-point in its saturation-curve, the relative sizes of said capacitor and said saturable inductor being such that said inductor requires something like a half-cycle of the electromagnetsupplying circuit to change its magnetization from substantially zero to the saturation-point in either polarity, and the capacitor being sufficient- 1y large to reactively store, each half-cycle, an amount of energy which is more than four times larger than the amount of energy which is drawn by the coil-energizing circuit during each halfwave when it is energizing the electromagnetcoil.

3. The invention as defined in claim 2, characterized by said phase-controlling means being a controllably saturated inductance, serially connected between-said electromagnet-supplying circuit and said square-wave voltage-producing means, said controllably saturated inductance having a saturation-controlling unidirectionalourrent winding and unidirectional-current means for controllably energizing said saturation-controlling unidirectional-current winding.

4. A delayed-firing rectifying device comprising an evacuated container having an anode and a cathode, circuit-lead terminals for a main rectifier-circuit using said rectifying device in transferring electrical energy between a main alternating-current circuit and a main direct-current circuit, said container comprising means for providing an intermediate arcing-space through which the arc must pass in playing between the anode and the cathode, electromagnet-means for at times establishing an arc-blocking magnetic field transversely across said intermediate arcingspaoe, said electromagnet-means having an energizing coil, and a time-controllable steep-wavefront electromagnet-energizing means for at times unidirectionally energizing said coil in an efiective amount which is sufiicient to block the play of an are between the anode and the cathode and for at half-cycle intervals reducing the strength of the electromagnet-field from an effective arc-blocking value to substantially zero at a percentage-rate of decrease which is considerably higher than the percentage-rate of decrease of the voltage at the end of each half-cycle in the main alternating-current circuit, said electromagnet-energizing means being so phased, relative to said main rectifier-circuit, that said electromagnet is deenergized at a predetermined time after said anode becomes positive with respect to said cathode, characterized by said timecontrollable steep-wave-front electromagnetenergizing means including the combination of an alternating-current electromagnet-supplying being a controlled tube, and

circuit, said electromagnet-supplying circuit including a phase-controlling means, a capacitor connected in parallel-circuit relation to said electromagnet-supplying circuit, means energized from the terminals of said capacitor for energizing two alternately pulsating unidirectional-current circuits having a common return-circuit, and means for providing a midpoint connection for said common return-circuit; each of said unidirectional-current circuits serially including a half-wave rectifier, one of said unidirectionalcurrent circuits serially including said electromagnet-coil, and said common return-circuit serially including an inductor of a magnitude sufiicient to maintain a fairly constant value of direct-current in said common return-circuit.

5. The invention as defined in claim 4, characterized by said means for providing a midpoint connection being a mid-tapped inductance-device energized in parallel-circuit relation to the terminals of said capacitor.

6. The invention as defined in claim 4, characterized by said means for providing a midpoint connection being the midpoint of two serially connected half-wave rectifiers, each of a type which becomes conducting substantially as soon as its impressed voltage becomes positive in the conducting polarity, said two serially connected half-wave rectifiers being connected in parallelcircuit relation to the terminals of said capacitor in such polarities as to complete the circuits for the respective unidirectional-current circuits.

7. The invention as defined in claim 4, characterized by each of said half-wave rectifiers, in the respective unidirectional-current circuits, being of a type which becomes conducting substantially as soon as its impressed voltage becomes positive in the conducting polarity, and further characterized by the energizing-circuit for said electromagnet-coil having serially included resistance which is at least about twice as large as the supply-frequency inductance of the CO1 18. The invention as defined in claim 7, characterized by the phase-controlling means being associated with the alternating-current electromagnet-supplying circuit.

9. The invention as defined in claim 7, characterized by the phase-controlling means being a controllably saturated inductance, serially connected between said electromagnet-supplying circuit and said capacitor, said controllably saturated inductance having a saturation-controlling unidirectional-current winding and unidirectional-current means for controllably energizing said saturation-controlling unidirectional-current winding.

10. The invention as defined in claim 4, characterized by each of said half-wave rectifiers, in the respective unidirectional-current circuits, controlling-means for causing each of said tubes to become conductmg at approximately the maximum-voltage point of its half-wave of impressed voltage from the terminals of said capacitor.

11. The invention as defined in claim 10, characterized by the conduction-commencing timephase of each of said controlled tubes being substantially fixed, and further characterized by the phase-controlling means being associated with the alternating-current electromagnet-supplying circuit.

12. The invention as defined in claim 10, characterized by the conduction-commencing timephase of each of said controlled tubes being sub- JOHN L. BUYER.

14 REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Weintraub Jan. 21, 1908 Jonas Dec. 14, 1915 Jonas Mar. 28, 1916 Weiller Apr. 21, 1936 McCoy May 22, 1945 

